In the automotive and appliance industries it is generally known to apply chemical bond coatings to parts fabricated of metal prior to applying a finish coat. Chemical bond coatings, such as phosphates, on metals are used to promote adhesion of finish coats, such as paint, varnish lacquer, and the like, to the substrate. Such phosphate coating additionally provides some underpaint corrosion protection. The chemical bond coatings are usually applied to individual component parts after fabrication from stock material, e.g., sheet steel, but in some cases such coatings may be applied to sub or complete assemblies. The components are frequently coated at the end of an assembly line prior to assembly and painting.
It is known in the art of coating metal substrates to first apply a chemical bonding coat, apply a film of paint or the like over the chemical bonding coat, and then to test the adherence of the paint film. The most generally accepted testing procedure involves the ASTM method designated as B117 Salt Spray Testing. Contingent upon a particular user's requirements the B117 testing method takes over 100 hours and may take as many as 336 hours to complete.
A problem which has plagued the art of painting metal substrates is that of finding a suitable and satisfactory technique for testing and assessing the propensity of a metal substrate to receive and adherently retain a paint film, i.e., before the paint is applied.
In the art of coating steel substrates it is known to use phosphates as a chemical bonding coat for a finish paint coat. Most phosphate coating quality control methods are based on visual inspections and until recently most of the industry has assumed that if a uniform and completely covering phosphate coating was present on the steel substrate the outer paint film would inherently adhere. Accordingly, previous testing procedures were provided for measuring the concentration of the phosphate coating, such as that disclosed in U.S. Pat. No. 3,899,364, or else for evaluating the integrity of the coating, such as that disclosed in Metals Handbook, 8th edition, Vol. 2, published by American Society for Metals, Metals Park, Ohio, page 544. The procedure described in the Metals Handbook involves the use of a special solution containing potassium ferricyanide and sodium chloride; a chromotography filter paper is dipped into the solution and placed on the surface to be evaluated; after five minutes the test paper is lifted and the phosphate exposed surface is inspected for blue color; the quality of phosphate covering is rated in accordance with the absence of or amount of blue color, with the most desirable result being no blue color.
While such integrity test is effective toward assessing whether a full coat has been applied to the substrate it is not effective toward assessing the "bite" or adherence of the coating with the substrate. It is this adherence with which the present invention is primarily concerned. The testing procedure of this invention provides a method through which the results of more time consuming paint film adhesion tests can be anticipated with a high degree of reliability. Such time consuming tests include those as described in U.S. Pat. Nos. 3,899,364 and 4,000,012 where the specimen panels are tested after the paint films are applied. U.S. Pat. No. 4,000,012 also discloses a presently accepted technique in the automotive industry for evaluating the adherence of paint on a steel substrate. This technique involves the above referred to ASTM method B117 Salt Spray Testing.